A ferroelectric crystal such as LiNbO3 and LiRaO3 is called a nonlinear optical crystal due to its nonlinear optical characteristics, and is used as a wavelength conversion device for converting the wavelength of light. Especially, the nonlinear optical crystal having a polarization reversed structure where polarization directions are periodically reversed and meeting the Quasi-Phase-Matching (QPM) condition has a large nonlinear coefficient and therefore has high wavelength conversion efficiency. Further, the nonlinear optical crystal can be applied to a wide wavelength range by changing the period of the polarization reversed structure.
As the types of the nonlinear optical crystal having the polarization reversed structure, the waveguide type in which a waveguide having the width of about several μm and having the polarization reversed structure is fabricated on the surface of a crystal, and the bulk type using the entire crystal and polarization reversion structure is formed in the entire thickness of the crystal have been studied.
The bulk-type nonlinear optical crystal can convert the wavelength of a beam having a larger diameter than that of the waveguide-type. Therefore, the bulk-type nonlinear optical crystal can receive laser light having higher energy and emit light having higher power. Further, the bulk-type nonlinear optical crystal can be aligned easily.
As one of the promising bulk-type nonlinear optical crystals capable of generating a visible light of a Continuous Wave (CW) having watt-level high power obtained based on a single path conversion of the fundamental wave, LiNbO3 (Periodically Poled LiNbO3: PPLN) having a periodic polarization reversed structure has attracted the attention. However, the LiNbO3 has a problem that the output becomes unstable due to the influences of, for example, optical damage and Green Induced Infrared Absorption (GRIIRA). Also, it is necessary to be heated to a high temperature to stabilize the operations.
Because of the disadvantages, MgO doped LiNbO3, namely, MgO:LiNbO3 has been studied. The MgO:LiNbO3 is expected to be used as the nonlinear optical crystal capable of outputting a watt-level CW light at room temperature with a single path configuration because of its better optical damage resistivity compared with that of LiNbO3.
For example, Patent Document 1 discloses a short-wavelength light source including an optical device having a single-polarized ferroelectric substrate, polarization reversed domains formed on the ferroelectric substrate, and grooves formed on the surface of the ferroelectric substrate. This short-wavelength light source may control the heat generated by the absorption of the harmonic wave generated when a fundamental wave is converted into a watt-level high-power harmonic wave.
Further, Patent Document 2 discloses an optical wavelength conversion device including a crystal having a nonlinear optical effect, periodic polarization reversed layers formed on the crystal, an incident surface formed on the end surface of the crystal, a radiating surface formed on the other surface of the crystal, and a metal film formed on at least a part of the surface or the rear surface of the crystal. This optical wavelength conversion device may control the change of the second harmonic output caused by the alternation of the refractive index due to the pyroelectric effect by the temperature change.    Patent Document 1: Japanese Patent Application Publication No. 2006-308731    Patent Document 2: Japanese Patent Application Publication No. 2006-106804